Single domain certificates represent the simplest type of certificates and they allow you to secure one and only one domain name. You cannot secure more domain names within the same certificate or add them later. It is as simple as this.
On the other hand, if the Certificate Authority that issues the certificate supports it, when you request a certificate for “your-domain.com”, your certificate will be automatically upgraded to secure both “your-domain.com” and “www.your-domain.com”. This is a feature that most of our single domain certificates have and is usually referred to as “securing both www and non-www domain name”.
